1. Field of the Invention
The present invention relates to a semiconductor memory device. It particularly relates to a column select line having a two layer structure and a clamping circuit which prevent malfunction when a column select line or a word line is disconnected by foreign matters or the like.
2. Description of the Background Art
FIG. 10 is a block diagram showing a configuration of a dynamic random access memory (referred to as a "DRAM" hereinafter), a conventional semiconductor memory device.
As shown in FIG. 10, the semiconductor memory device is provided with subarrays 1 into which a memory cell array is divided, a row decoder 2 arranged at each subarray 1 for selecting a word line WL, and a column decoder 3 arranged at each subarray 1 for selecting a column select line CSL.
Furthermore, the semiconductor memory device shown in FIG. 10 is a DRAM .times.4 configuration and has a memory cell array consisting of four subarrays 1. Each subarray 1 has a plurality of word lines WL including a redundant word line, a plurality of pairs of bit lines (not shown) arranged to cross the plurality of word lines WL and including a redundant pair of bit lines, a plurality of column select lines CSL arranged parallel to the plurality of pairs of bit lines and including a redundant column select line for sending a signal for selecting a pair of bit lines, and memory cells (not shown) connected to intersections of ward lines WL and the pairs of bit lines.
One end of column select line CSL is connected to column decoder 3 and the other end is opened. Thus, when column select line CSL is disconnected due to foreign matters or the like, a redundant column select line is substituted for the column select line CSL while the disconnected portion is floating, causing multiselection of memory cells and hence malfunction of the device.
Furthermore, a word line WL also has one end connected to row decoder 2 while the other end open. Thus, when word line WL is disconnected due to foreign matters or the like, a redundant word line is substituted for the disconnected word line WL while the disconnected portion is floating, causing multiselection of memory cells and hence malfunction of the device.